The Times of your Life


The basic unit of time is the second. The second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of cesium 133. The UT times (based on the terrestrial rotation), with a day of length 86400 seconds (by definition), are kept in line with the occasional addition of leap seconds.

Solar Time

Times based on the Sun include :

Julian Days

Astronomers realized a long time ago that the civil time system is cumbersome for determining lengths of intervals. Just how many days have passed since July 4, 1776? To get around some of these dificulties, astronomers generally use Julian dates. The Julian date is the number of ephemeris days elapsed since 12h UT on 1 January, 4713 BC (which is -4712 AD). The Julian date on January 1.5, 2000 will be 2,451,545.0. This is defined as J2000.0. Note that the JD increments at 12h UT (Greenwich Mean Time).

Because the JD is a large number, you will often see the Reduced Julian Date (JD-2,400,000) used. The modified Julian Date (MJD) is also commonly used. MJD=JD-2,400,000.5. Note the extra 0.5 day in the definition. Modified Julian dates increment at the same time as Universal Time. Some have been known to use HJD=JD-0.5. Be sure you know what convention is being used. Times in the Batten catalog are in Reduced Julian Days.

To convert the civil time into the Julian day, use the following algorithm

JD = fix(365.25*f) + fix(30.6001*(g+1)) + d + A + 1,720,994.5
where y is the year, m is the month, d is the day of the month,
f=y for m>2 and f=y-1 for m<3
g=m for m>2 and g=m+12 for m<3
A=2-fix(f/100) + fix(f/400).
This algorithm works for Gregorian civil calendar dates only.

When you compute with Julian dates, be sure to use double precision floating point arithmetic.

The IDL procedure JULDATE will return the reduced Julian date. In IDL, type juldate,jd, and you will be prompted for input.

Sidereal Time

Sidereal Time is based on the Earth's sidereal rotation period (rotation with respect to the stars). The length of the sidereal day is 24 sidereal hours, or 23h56m4s Solar time. The difference is due to the angular motion of the earth around the Sun. In a year there is one more sidereal day than Solar day. The sidereal time is a measured locally. The local sideral time (LST) is the right ascension of the zenith. The hour angle HA of an object with right ascension RA is given by HA=LST-RA Right ascension is generally measured in units of time, not degrees of arc. 24 hours of time corresponds to 360 degrees of arc at the equator. Therefore, where <em>d</em> is the declination of the target. You can use analogous formulae to convert degrees of arc to distances on the earth's surface.

Sidereal Time

The basic reference for all this is the Astronomical Almanac, available in the reference section of the library. It is published annually.

Some Web sites: